It is necessary to adjust the operation of the breaker contacts with respect to the axial position of the distributor cam shaft to adjust the ignition timing. The adjustment parameter is usually provided by sensing the static vacuum in the carburetor or inlet manifold of the internal combustion engine. This vacuum is dependent on the instantaneous position of the throttle. It is highest at about half throttle opening. The breaker contact support plate is linked by means of a rod to a spring-loaded membrane of a diaphragm chamber in such a manner that, upon change in the vacuum, the breaker support plate rocks coaxially with respect to the distributor shaft about a small angle so as to vary the relative position of the breaker contact with respect to the land of the cam shaft of the distributor.
Movement of the breaker support plate should be free, so that the contacts located thereon can operate in accordance with commanded timing; for free operation, it is important that the breaker support plate is secured by a bearing which permits twisting or rotory motion without, however, introducing or permitting play in radial or axial direction.
It has previously been proposed to construct a breaker timing adjustment arrangement by securing an extended bearing to the main support plate of the distributor and forming a collar or flange on the breaker support plate, the telescoped connection of elements forming the bearing. Radial play of the bearing is then determined by manufacturing tolerances. To eliminate play, a ball is pressed against the breaker support plate by a bail-shaped holder formed as a spring. The ball eliminates axial play without interfering with rocking movement of the breaker support plate about the axis of rotation of the distributor cam. This arrangement satisfies the requirements of eliminating axial and radial play; it is, however, important that the tolerances in manufacture are small. The manufacture of the parts is usually effected by drawing. The bail-shaped spring and the ball, which bears against a location on the breaker support plate which is relieved to seat the ball, require additional assembly and manufacturing steps which increase the cost of the final article -- a cost which should preferably be avoided.
It has also been proposed to eliminate axial play when inserting a breaker contact support plate into an elongated bearing by providing an axially acting spring secured to a ring. This still requires an accurate elongated sleeve bearing and a ring of accurate size, all parts which have to be accurately made and accurately mounted in order to ensure reliable, troublefree operation. Incorrect mounting, excessive tolerances or careless assembly lead to malfunction.